We have demonstrated the non-cooperative character of hormone-receptor interactions and the nearly globular structure of membrane receptors (previously thought to be highly elongated), and have developed a structural model for the binding unit and subunits of the lymphocyte insulin receptor which includes the possibility of a dimeric structure containing two insulin binding sites (divalent). In order to extend these and other methods to the study of the structural-functional relationships central to the elucidation of the mechanism of insulin action, we have investigated the continuously-cultured, spontaneously-differentiating muscle cell line, BC3H-1. We have shown that during spontaneous differentiation to nonfusing myocytes, these unique cells develop high affinity insulin receptors and insulin-stimulated responses to physiological concentrations of the hormone, including insulin-stimulated glucose uptake and changes in cell membrane potential. We therefore propose to investigate the structure of this functional insulin receptor, its subunits and its binding valence. The mechanisms of the observed insulin-induced changes in membrane potential will be studied with respect to changes in ion fluxes and energy-dependent ATPase's. Insulin-stimulated glucose transport will be investigated by examination of the target size of the glucose carrier (radiation inactivation analysis), the temperature dependence and kinetics of hexose transport, and their possible relationship to insulin-stimulated changes in membrane potential or membrane fluidity. The contribution of receptor synthesis, degradation, or internalization to the development and regulation of insulin receptors during myocyte differentiation or "down regulation", and their functional consequences for insulin-stimulated responses, will be determined. Finally, the structural and functional relationship between insulin receptors and insulin-like growth factor receptors will be examined to localize the points of coupling between the biochemical effects common to both hormones. It is intended that these results will better define the chains of events that comprise the multiple cellular responses to insulin in muscle, its quantitatively most significant target tissue.